This invention relates to a rolling bearing which is used with fitting stress applied to the inner race made of alloy steel (the stress being the average of the tensile force in the section of the inner race which is applied to the latter circumferentially thereof), and in which the inner race is long in service life and is simultaneously prevented from being cracked even when used under high fitting stress.
In addition, this invention also relates to a rolling bearing which is long in service life even when used with a foreign-matter-mixed high-temperature lubricant, and in which the inner race is high in dimensional stability.
For instance, an automatic self-aligning roller bearing used for a paper machine is sometimes used under a high fitting stress over 100 MPa, to prevent the inner race and the shaft from creeping. In order to apply the fitting stress with ease, the inner race whose inner cylindrical surface is tapered, is press-fitted on the tapered shaft. In general, the inner race thus shaped is made of completely hardened steel such as high carbon chromium bearing steel (containing about 1% C by weight and about 1.5% Cr by weight) which is hardened and tempered.
When the bearing is used with a fitting stress over 100 MPa applied to the inner race of completely hardened steel, the fitting stress and the rolling stress in combination may crack the inner race axially if a non-metallic foreign matter is located near the raceway track.
In order to eliminate this difficulty, heretofore the following methods are employed which are based on the general technical knowledge that it is most effective to increase the compression residual stress of the raceway track or to increase the fracture toughness of the material itself: That is, the completely hardened steel is subjected to austempering, or carburized steel is employed, to increase the compression residual stress of the raceway track.
However, the above-described methods suffers from the following problems:
That is, the method of austempering the completely hardened steel is disadvantageous in the following point: With the method, the compression residual stress which can be applied to the raceway track is about -100 MPa. Therefore, in the case where the inner race is used under a high fitting stress higher than 130 MPa, it is impossible to prevent the inner race from being cracked.
The method of employing the carburized steel is effective in preventing the cracking of the inner race which is used under a fitting stress over 130 MPa, because a compression residual stress of the order of -200 MPa may be applied to the raceway track by controlling the conditions of carburizing, hardening and tempering. However, the method is still disadvantageous in that, in the case where a steel material small in carbon content, about 0.20% by weight, is carburized, the carburizing operation takes a relatively long time period.
On the other hand, the carburizing time is proportional to the square of the carburizing depth. Hence, middle and large bearings requiring a deeper carburized layer are lower in productivity; that is, they are unavoidably higher in manufacturing cost.
Further, when the inner race which is used under a high fitting stress is particularly used in a high temperature lubricant mixed with foreign matters (hereinafter referred to as "foreign-matter-mixed high-temperature lubricant", when applicable), there is a problem in which the dimensional stability and the service life thereof is further getting worse.
When high carbon chromium bearing steel or carburized steel is tempered at 230.degree. C. or higher, the amount of residual austenite is decreased, so that its dimensional expansion with time is prevented, while the martensite is sufficiently tempered, so that its dimensional contraction with time is prevented; however, the surface hardness is decreased. Hence, when an inner race which is heat-treated as described above is used to fabricate a rolling bearing, the rolling bearing is insufficient in the hardness of the raceway track, and its rolling life is decreased when used with foreign-matter-mixed high-temperature lubricant.